1. Field of the Invention
The present invention relates to image forming. More particularly, the present invention relates to an image forming apparatus and an image forming method that form a multi-beam latent image.
2. Description of the Related Art
Along with the improvement of function of an image forming apparatus, an image forming speed (PPM: prints per minutes) per a unit time of the image forming apparatus also increases. In recent years, an image forming apparatus that utilizes a surface emitting laser (hereinafter, VCSEL (Vertical Cavity Surface Emitting LASER)) to perform multi-beam exposure is proposed to form an image at higher speed and with higher resolution. Moreover, an image forming apparatus that performs duplex printing is provided in response to the request of resource saving.
For this reason, along with the improvement of an imaging speed, an automatic duplex printing apparatus has a trend that a time interval from the record of the first surface to the record of the second surface of paper is shortened. For example, a high-speed model can perform the print of the first surface and the print of the second surface within ten seconds. Because a carrying distance from heat fixing corresponding to the record of the first surface of paper to the record of the second surface is likely to be shortened in conjunction with the miniaturization of the image forming apparatus, a time for which the paper is not exposed to a high-temperature portion is shortened. Besides a time interval, printing paper further comes under thermal influence and thus is hard to get cold.
When two-sided recording is performed in such a situation, it is known that images that are printed on the first and second surfaces, which are both sides of paper, have the difference of magnification of 0.2% to 0.4% due to the change of heat and moisture when a sheet of premium grade paper having the thickness of about 80 micrometers is used as printing paper.
In regard to the problem described above, a technology for providing a sub-scanning magnification zooming function to an image forming apparatus, and performing reduction print by the thinning of sub-scanning image data and expansion print by the addition of image data have been conventionally known as disclosed in, for example, Japanese Patent No. 3373266.
The fluctuation of magnification might be able to be cancelled by the method disclosed in Japanese Patent No. 3373266. However, along with the high resolution of an image to be formed, a periodic image that forms, for example, one line in every five lines has problem occurring globally such image defect as concentration unevenness or moire when a single line is thinned or added from or to the image for correcting the fluctuation of magnification.
Moreover, it is necessary to prevent banding caused by the number of screen lines, the interference of the magnification, and the like along with the cancellation process of the fluctuation of magnification.